I noticed that when I have a project whose source files are on a network-share (path is off of /Volumes/...), the symbol indexing doesn't appear to work. Looking at ~/Library/Application Support/Sublime Text 3/Index, theres' an 000003.log, but it doesn't grow beyond 64K. When I tried copying the project files locally and making that into a Sublime project, the index file immediately grew to a couple of megs (and I had a long list of symbols when using Goto Symbol in Project).